1. Field of the Invention
The present invention relates to a cleaning method for cleaning a substrate having a porous structure in the surface thereof and, more particularly, to a cleaning method of porous surface. The present invention is suitably applicable as a cleaning method for cleaning a porous silicon semiconductor substrate used for selective etching or dielectric isolation of a semiconductor or used as a light emitting material, which demands the most strict control of cleanliness. The invention also relates to a cleaning method of a semiconductor surface.
2. Related Background Art
The method for forming the porous structure typified by porous silicon was introduced by A. Uhlir in 1956 (Bell. Syst. Tech. J., 35, pp. 333).
After that, application technologies were developed including use thereof as a selective etching layer or as an isolation area after oxidized, epitaxial growth on porous silicon, etc. The present applicant disclosed in Japanese Laid-Open Patent Application No. 5-21338 that an SOI (Silicon on Insulator) substrate was fabricated using a single-crystal silicon thin film epitaxially grown on porous silicon.
In recent years, the photoluminescence phenomenon of porous silicon was discovered. It is since drawing attention as a self-radiative material utilizing not only its structural features but also its physical properties.
A popular method for forming porous silicon is anodization in an electrolyte solution of a mixture of hydrofluoric acid/pure water/ethanol by the conventional electrochemical cell structure. Since many dust particles adhere to this porous silicon, it is better to remove the dust particles by cleaning before epitaxial growth on the porous silicon. Conventional cleaning comprises only rinsing the above electrolyte solution inside pores with pure water. There is no example of introducing a positive cleaning method of the surface.
It is well known that, cleaning is indispensable before and after processing in the semiconductor processes and that it is also unavoidable in the case of the porous silicon substrate. The conventional cleaning methods of bulk substrate (non-porous substrate) include chemical wet cleaning with a combination of chemicals such as sulfuric acid/hydrogen peroxide, ammonia/hydrogen peroxide, hydrochloric acid/hydrogen peroxide, or hydrofluoric acid/pure water, as typified by RCA cleaning (RCA Review, 31, pp. 187-205, 1970) developed by W. Kern et al., which is said to be a method effective for removal of dust particles on the surface.
Recently, Ojima et al. (Research Report, Institute of Electronics, Information and Communication Engineers, SDM95-86, ICD95-95, pp. 105-112, July 1995) proposed a method for removing the dust particles by applying a high-frequency ultrasonic wave of about 1 MHz (megasonic wave) to the bulk substrate in a mixture of hydrofluoric acid/hydrogen peroxide/pure water/surfactant or in ozone-added pure water, for the purpose of decreasing amounts of cleaning chemicals.
This method is characterized by cleaning conducted in such a way that the silicon substrate is oxidized with hydrofluoric acid and hydrogen peroxide to be etched, the dust particles on the surface are lifted off from the substrate, and potentials of the dust particles are neutralized with the surfactant to prevent re-deposition of dust particles on the substrate. The purpose of this method is to give energy upon the lift-off of dust particles and to remove organic matter attached to the surface of substrate by generation of ions from the pure water by megasonic wave application. Thus, the basis of the cleaning is chemical cleaning. Use of the ozone pure water is for the purpose of enhancing the organic removing effect.
In ultrasonic cleaning, cleaning with low frequencies ranging approximately from several ten kHz to 400 kHz conventionally used is "liquid resonance cleaning" to remove the dust particles of several ten pm on the substrate surface by applying a strong shock wave to the substrate surface by liquid cavitation (expansion/compaction) due to the liquid resonance action. In contrast, cleaning with high frequencies ranging from 800 kHz to 1.6 MHz is "sound-wave scrub cleaning" to remove the dust particles by giving kinetic energy based on resonance to the dust particles, which enables the removal of dust particles of submicron order without damaging fine patterns.
Low-frequency cleaning causes damage to fine patterns due to the cavitation impact and is no longer used in the semiconductor processes of 4-Mbit DRAM and processes. On the other hand, high-frequency cleaning is recognition as a method capable of cleaning fine dust particles without damaging the patterns.
According to the experience of the present inventors, the substrate having the surface of the porous structure is of fine and dense structure and has long pores. Therefore, use of chemicals in conventional chemical wet cleaning causes the chemicals to intrude deeply into the pores, which makes complete elimination of chemicals difficult even with rinsing with pure water for a long time. It negatively affects the post-processes like the epitaxial growth on the porous structure.
If the physical removal of dust particles is attempted by superposing the conventional ultrasonic of low frequency on pure water, the fragile will raise the porous structure will raise the may collapse due to the sound pressure of shock wave of cavitation even in the relatively high frequency region around 200 kHz.
This problem results from the structure of porous silicon, and the experience of the present inventors is not peculiar. The reason why the positive cleaning of porous silicon surface has not been conducted heretofore is believed to be the above problem.
In addition, it was found by the present inventors that when the surface of the porous silicon substrate was rinsed with pure water after formation of the porous structure by anodization, several hundred dust particles not less than 0.3 .mu.m, obtained from laser reflection intensity distribution, adhered to the surface in a 5-inch-diameter wafer as shown in FIG. 28. In the bar graph, classification of L1, L2, and L3 indicates rough classification of sizes of dust particles obtained from laser reflection intensities from the dust particles, and the sizes increase in the order of L1&lt;L2&lt;L3.
The number of dust particles adhering upon anodization gradually decreases with increasing number of batch of anodization in the single wafer process as shown in FIG. 28, because the dust particles in the liquid decrease as captured by the substrate. Such high numbers are, however, anomalous numbers when compared with those in the current semiconductor processes, wherein the dust particles are removed down to several or less particles on the surface of a bulk substrate after RCA-cleaning.
These dust particles adhering during anodization can be decreased to some extent by liquid circulation of the above electrolyte solution and collection of dust particles with a filter, but the decrease is not sufficient yet. Conceivable causes of adhesion of dust particles include dust particles mixed in an anodization system and in the electrolyte solution and dust generated from workers during the process. Further, it is also conceivable that the surface of porous silicon becomes hydrophobic because of the anodization in the high-concentration hydrofluoric acid electrolyte, so that the silicon substrate tends to be electrostatically charged, thereby attracting the dust particles. Therefore, prevention of adhesion of the dust particles is not easy.
As a matter of course, such dust particles the generate imperfections in the subsequent processes, such as anomalous growth or pinholes in the film-forming process, and were problematic in applications of porous silicon.